I'm interested in getting a battery to run my charger from at the field, because the constant use of my car battery for this purpose is hurting it. What I'm looking for is the best value for the money. I'd prefer to spend 40$ or less. I'd prefer gell cell to wet cell. Any ideas?

i got a deep-discharge marine battery at costco for about $45. perfectly suited to the job. i actually killed my car battery charging packs off it, and replaced it with the marine battery. the marine battery has plenty of juice to start the car, and it should not be hurt by frequent discharging.

Careful, somewhere I heard, read or was told that a marine battery doesn't get along with an automobile system.

I keep my 'marine' in a battery box just on the outside wall of my 'little corner of the house'. Got a cable going in thru the wall and a female plug on the end of it, with a matching male on my charger for 'home charging'.
On this same cable(2-10 gauge wires/wrapped), 12" to 16" from the box and just before it goes in thru the outside wall is another male/female plug setup(just like the inside). Unplug this plug, toss the box in the truck and SECURE IT IN PLACE(you think the zagi battery can do some damage ).

Problem is, the units like these I have seen are more than 40$. More like 60$-100$. I heard someone mention the gel cell batteries used in motorized wheelchairs, but the ones of those that I have seen are pricey too.

Can't think of a reason not to put a marine battery in a car. The one in my boat charges off a V-8 that looks like it has the same charging system as a car. My understanding is that the marine or deep cycle battery has thicker plates so they don't warp when completely discharged (not a good idea for a car battery as Ben found out).

Any Deep- Cycle should not be used in a car. They are made to Discharge slowly and to charge slowly. A car battery is made to give out high cranking amps when you start your car and will charge fast. Most cars of today have a 100amp Alternator but the battery won't charge anywhere near that. Someone I know went thru two deep-cycles before he got smart. Some will get away with it some won't. If you have a small 4cly. maybe and 8 cly probably not for long and some real cold morn you will be walking to work. But the Marine battery is just a heavy duty car battery where the deep-cycle is more for your elec trolling motor and will run it all day or run you RV lights when at camp

There is no reason whatsoever that a marine battery won't function correctly in a car.
I use a 12 volt 7A maintenance free battery that is relatively small, lightweight and very portable. Totally meets my needs, which are relatively lightweight. Cost me $17.00 new.
Deciding what size battery YOU need depends on what you're charging. What are your typical daily needs?
Finding a marine battery within your budget will be difficult, if not impossible. They aren't cheap, but then you get what you pay for.
Look at Costco and Sears.

I agree a marine battery will work in a car just don't mix up marine with deep-cycle they are two different animals

Actually, they are and they aren't.
Marine batteries are of two types. Deep cycle marine batteries and starting marine batteries.
Deep Cycle Batteries are used where discharging and charging occurs frequently.
Batteries without substantial materials designed for true deep-cycling will lose more than half of their capacity after only a few cycles. A 200 Amp-hour battery will shortly become a 100 Amp-hour battery for the remainder of its shortened service life. What initially may seem to be an inexpensive battery to purchase, now costs twice as much per Amp-hour. True Deep cycle batteries will perform well as cranking batteries, however, cranking batteries will not survive deep cycle use.
Deep cycle batteries can be used in any application and exhibit a long service life, while cranking batteries are limited to starting applications only. Cranking batteries exhibit poor service life in cycling applications.